Use of 'it' when referring to people.
According to the Webster dictionary (www.webster.com) the pronoun 'it' can be used in reference to "a person or animal whose sex is unknown or disregarded <don't know who it is>."
"It" is often used when talking about babies or children or in sentences like "It is me". But can that pronoun always be used when a reference to the person's sex is to be avoided? That is, instead of using constructions like "he/she" or "his/her", is it correct to use "it" or "its"?
Just so you know, to use "it" to refer to a person other than a baby is very impolite, besides limited usages such as "it is me", where "it" isn't referring to another person, but rather whoever/whatever is announcing their present, which just happens to be the speaker. Rather, one should use "they" when referring to people in such a fashion. Note that while some prescriptive sources will say that one has to use "he", in many dialects "he" *only* refers to male people and "higher" animals, and its use in such a fashion will almost certainly come off with a general air of being pedantic (as few in practice actually use "he" in such a fashion, in at least North American English) and to a degree sexist.
As Travis noted, we do not use "it" to refer to persons, except for "baby" or "infant" or "child." As you stated, we do use "it" for animals.
Travis is also spot on about "they" which is a perfectly acceptable solution to get around the "he/she" conundrum.
And again, to echo Travis, don't be bamboozled by pedantic grammarians who "insist" you cannot use "they" because it's third person plural - we've been using "they" this way in English for centuries.
Jane Austen used it. Will Shakespeare used it.
Works for me!
However, you can use the word 'it' to refer to a gender-unknown person in a situation like "who is it?" on the phone or at the door.
You wouldn't use "it" to refer to a person unless you are Gollum or Buffalo Bill from Silence of the Lambs.